1. Field of the Invention
The present invention relates to a grid apparatus for a color cathode ray tube which eliminates vibration of the grids and to a method of making such grid apparatus.
2. Description of Related Art
In conventional grid apparatus for a color cathode ray tube, aperture grills are known such as disclosed in Japanese patent publication Nos. 59 (1984)-18825 and 59 (1984)-18826. In such a grid apparatus, as shown in FIG. 4, a frame 5 comprises a pair of support bars 1, 2 disposed in parallel to each other with a predetermined space therebetween, and resilient support members 3, 4 which are substantially U-shaped are attached between the ends of the support bars 1, 2. A multiplicity of parallel spaced ribbon-shaped grid elements 6 are attached after being stretched with the required tension and at a predetermined pitch between the two support bars 1, 2 as shown. In making the grid apparatus 7, the grid elements 6 are welded to the frame 5 when they are stretched and deformed by pressure. Then they are welded after which the pressure which placed then in tension is released. Thus, the grid elements 6 are in a stretched condition and then they are heat-treated to blackened then in a temperature range of about 450.degree.to 470.degree. C.
However, thermal creep occurs in the grid apparatus 7 during the blackening process because the high temperature causes the stretched grid elements 6 to be expanded. As stated above, the grid elements 6 were welded to frame 5 before the blackening process. Also, spring-back of the frame 5 causes the tension to become lower in the grid elements 6 after the heat treatment than the tension was prior to the heat treatment because of the permanent deformation of the frame. Although the thermal creep occurs principally during the blackening process in the heat treatment, it also can occur at the time when frit sealing of a panel and a funnel in the heat treatment (at 440.degree. C. or so) is done.
Therefore, when a television receiver with a color cathode ray tube is placed in operation after completion of the above processes, the grid elements 6 of the grid apparatus 7 will vibrate which is caused by sound in the television set which generates image noise. One of effective means for reducing such image noise is to maintain a higher tension of the grid elements 6. However, since the tension applied prior to the heat treatment is very close to the breaking strength limit of the grid elements 6, the tension cannot be further increased.